1. Field of the Invention
The present invention relates to vector network analyzers in general and in particular to a method for eliminating subharmonic false locking in a sampler and frequency multiplier based phase-locked-loop signal source locking system which is used for generating an r.f. signal for use in such analyzers.
2. Description of the Prior Art
Vector network analyzers require an accurate and stable signal source to generate the r.f. signals used for testing and analyzing the electrical characteristics of an electrical circuit. When being analyzed the circuit is commonly called a device-under-test or DUT.
A sampler based source locking system, of the type used as a signal source in a vector network analyzer, allows an open loop source with a maximum frequency error of .+-.F.sub.error to be phase-locked to a more accurate and stable reference signal. For example, in a typical prior known sampler based source locking system there is provided a source comprising a voltage-controlled oscillator (VCO) having an output coupled to a frequency multiplier for providing an output signal having a nominal fundamental frequency F.sub.s. The output of the source is coupled to a sampler which is driven by a first local oscillator having a frequency F.sub.L01. On the output of the sampler there is produced a first intermediate frequency signal having an intermediate frequency IF.sub.1 which is equal to the difference between the frequency of the source F.sub.s and a harmonic frequency or comb line of the first local oscillator. For convenience, the harmonic frequency is often defined by a harmonic number (H). The harmonic number H is the integer by which the frequency F.sub.L01 of the oscillator is multiplied to obtain the harmonic frequency, e.g. 531.75 MHz.times.H=60087.75 MHz, where H=113.
The first intermediate frequency signal IF.sub.1 is then fed to a mixer and mixed with the output of a second local oscillator to produce a second intermediate frequency signal having an intermediate frequency IF.sub.2. The output of the mixer is then fed to a phase detector along with a reference signal having a frequency F.sub.ref. The output of the phase detector, which comprises an error signal having a magnitude corresponding to the difference between IF.sub.2 and F.sub.ref, is then used to control the frequency of the VCO so as to maintain the second intermediate frequency IF.sub.2 at F.sub.ref.
Alternatively, the mixer stage may be omitted. In this event the first intermediate frequency IF.sub.1 is compared in the phase detector with an appropriate alternative reference frequency F.sub.ref to maintain the frequency IF.sub.1 at the alternative reference frequency F.sub.ref.
Heretofore it has been the practice to select for the first local oscillator the lowest usable harmonic number (H) and the highest usable local oscillator frequency F.sub.L01 in order to maximize sampler efficiency and minimize local oscillator phase noise due to frequency multiplication. A typical algorithm which has been used for this purpose is as follows
______________________________________ 10 F.sub.1 = INT (20 .times. F.sub.s) 20 H = 1 + INT (F.sub.1 + 1788)/10730 30 Z = 5365/(3 + H/1250) 40 A = 1 + INT [(F.sub.1 + Z)/H] 50 K = (H .times. A - F.sub.1 + 45) 60 F.sub.L01 = A .times. .05 70 F.sub.L02 = K .times. .05 80 IF.sub.1 = H .times. F.sub.L01 - F.sub.s ______________________________________
wherein
INT is the integer value of the parenthetical terms associated therewith PA1 F.sub.s is the fundamental output frequency of the source PA1 H is the harmonic number of the first local oscillator PA1 F.sub.L01 and F.sub.L02 are the frequencies of the first and second local oscillators PA1 IF.sub.1 is the first intermediate frequency and PA1 F.sub.1, Z, A and K are parameters used in calculating the above quantities
Using the above algorithm and assuming a fundamental source frequency F.sub.s of 60 GHz, i.e. 60000 MHz, the lowest harmonic number H, the highest first and second local oscillator frequencies F.sub.L01 and F.sub.L02, respectively, and the first intermediate frequency IF.sub.1 in megahertz which can be used to lock the source within its search range of .+-.F.sub.error, typically .+-.60 MHz, is calculated as follows:
______________________________________ 10 F.sub.1 = INT (20 .times. F.sub.s) F.sub.1 = INT (20 .times. 60000) F.sub.1 = 1200000 20 H (HARMONIC NUMBER) = 1 + INT(F.sub.1 + 1788)/10730 ##STR1## ##STR2## H = 1 + INT (112.0026) H = 1 + 112 H = 113 30 Z = 5365/(3 + H/1250) Z = 5365/(3 + 113/1250) Z = 5365/(3 + 0.0904) Z = 5365/3.0904 Z = 1736.02 40 A = 1 + INT [F.sub.1 + Z)/H] A = 1 + INT [(1200000 + 1736.02)/113] A = 1 + INT [10634.83] A = 1 + 10634 A = 10635 50 K = (H .times. A - F.sub.1 + 45) K = (113 .times. 10635 - 1200000 + 45) K = (1201755 - 1200000 + 45) K = 1800 60 F.sub.L01 = A .times. .05 =10635 .times. .05 F.sub.L01 = 531.75 MHz 70 F.sub.L02 = K .times. .05 F.sub.L02 = 1800 .times. .05 F.sub.L02 = 90 MHz 80 IF.sub.1 = H .times. F.sub.L01 - F.sub.s IF.sub.1 = 113 .times. 531.75 - 60000 IF.sub.1 = 60087.75 - 60000 IF.sub.1 = 87.75 ______________________________________
If the source uses a frequency multiplier to generate the final r.f. energy, as many of them do, its output will also contain subharmonic frequencies which will mix with lower harmonic comb lines of the sampler. This can result in subharmonic false locking of the phase-locked-loop. For example, a source comprising a frequency tripler having a nominal fundamental output frequency F.sub.s of 60 GHz, i.e. 60000 MHz, a maximum frequency error signal, i.e. search range, of .+-.60 MHz, a first local oscillator frequency F.sub.L01 of 531.75 MHz and a first intermediate frequency IF.sub.1 of 87.75 as calculated above, can, during searching for a lock, shift through a frequency range of from 59540 MHz to 60060 MHz. When this is done, it has been found that a false lock can occur at an output frequency F.sub.s of 59953.5 MHz producing a subharmonic false lock error of 46.5 MHz which is clearly within the permissible search range, i.e. .+-.60 MHz of the source. This is due to the fact that the second subharmonic of the source, i.e. 39969 MHz, will mix with the 75th harmonic, i.e. H=75, of the first local oscillator, i.e. 39881.25 MHz, in producing the intermediate frequency IF.sub.1 of 87.5 MHz.